The Next Graduate: Clandestine(?) extra-departmental edition

It is Friday evening. A Master's student in a discipline in which I hold an undergraduate degree has asked me to be his language advisor. English is the student's fifth or sixth language. The student reports that his departmental supervisor lacks sufficient mastery of the English language to guide the him on language matters. The student recognises that his skills with tense and conjugation could be improved. I am to appear as an acknowledgement in the thesis.

I have tentatively agreed to the proposal, and will likely start receiving chapters shortly. The Master's thesis is to be approximately 70 pages (front matter, body, and back matter), to be completed in the next few months. The student and I understand that I will help through teaching by reviewing manuscripts and discussing reasons behind potential edits. Although I have some knowledge in the scientific subject matter, I am not qualified to evaluate the student's new research in this area. At most, I would try to ensure that the argumentation is logical, as I probably do not have time to gain a substantive understanding of the details. I have known the student for four months now in various social contexts. He is a solid individual.

When I meet with the student tomorrow, I will suggest that he or we discuss my role in this with his current supervisor, since this kind of editing/revision will affect the student's timeline. I don't know how the norms for supervision in my own department may be similar or different from the norms in the student's department.
I also do not want the supervisor to be offended by this arrangement. I am undecided as to how I would frame this arrangement for my own department, since we don't have the bureaucracy or accounting to recognize an extra-departmental language advisor role or activity.

I look forward to being able to focus on the finer points of scientific argumentation, rather than on strictly supervisory responsibilities!